Bearings configured to support main shafts of turbo machines, such as a gas turbine and a turbocharger, are required to endure severe environments involving high temperature and high speed rotation. Attention has been focused on a foil bearing as a bearing suited to use under such conditions. The foil bearing has bearing surfaces formed of flexible thin films (foils) having low flexural rigidity, and is configured to support a load by allowing the bearing surfaces to be deflected. During the rotation of the shaft, fluid films (such as air films) are formed between an outer peripheral surface of the shaft and the bearing surfaces of the foils, and the shaft is supported in a non-contact manner.
A reaction force of an airflow generated by high speed rotation of the turbine in thrust directions is applied to the shafts of the gas turbine and a supercharger. Therefore, the shafts are required to be supported not only in a radial direction but also in the thrust directions. For example, in Patent Literatures 1 to 3, there is described a leaf-type thrust foil bearing as a type of thrust boil bearings configured to support a rotary member (thrust collar) in the thrust directions. The thrust foil bearing includes a plurality of leaves provided at a plurality of circumferential positions on an end surface of a disc-like foil holder. One circumferential end of each of the leaves is set as a free end, whereas another circumferential end of each of the leaves is fixed to an end surface of a fixing member. When the rotary member is rotated, a thrust bearing gap is formed between a bearing surface of each of the leaves and an end surface of the rotary member, which is opposed thereto. With a fluid film in the thrust bearing gap, the rotary member is supported in the thrust directions in a non-contact manner.
In the thrust foil bearing described above, the leaves are provided so as to be separated from each other in the circumferential direction. Therefore, regions between the leaves in the circumferential direction do not function as the bearing surfaces, resulting in a fear of insufficient supportability. Further, in the thrust foil bearing described above, a back foil (bump foil, spring foil) is provided below the leaves so as to apply an elastic force to the leaves, thereby adjusting the gap (thrust bearing gap) between the bearing surface of each of the leaves and the rotary member. By providing the back foil as described above, however, the number of components is increased, resulting in higher cost.
For example, in a thrust foil bearing 310 described in Patent Literature 4, an end portion 312a of each of foils 312 on one circumferential side (on a downstream side in a rotation direction of a shaft) is set as a free end, as illustrated in FIG. 42. The end portion 312a is arranged so as to be overlapped on adjacent one of the foils 312. A region including the end portion 312a of each of the foils 312 on the one circumferential side forms a top foil portion A′ having a thrust bearing surface S1′. A region including an end portion 312b of each of the foils 312 on another circumferential side forms a back foil portion B′ configured to support the top foil portion A′ of the adjacent one of the foils 312 from behind (lower side in FIG. 42).
As described above, by overlapping the adjacent foils 312, the thrust bearing surfaces S1′ can be provided continuously over the entire periphery. At the same time, the back foil can be omitted to reduce the cost. Further, in the thrust foil bearing, radially outer rims of the leaves are coupled by a ring-shaped coupling portion, and the coupling portion is fixed to a foil holder. Therefore, below the leaf that forms the bearing surface, a fixing portion between another one of the leaves and the foil holder is not provided. The leaves are fixed to the foil holder with a fixing member, through welding, or by other methods. Hence, concavity and convexity are generally formed at the fixing portion. By the absence of the fixing portion below the bearing surface as described above, it is possible to prevent a risk of adverse effects of the concavity and convexity of the fixing portion on the bearing surface so as to prevent reduction in supportability.
Further, in Patent Literature 5, there is described a foil bearing (radial foil bearing) including foils 512 as illustrated in FIG. 53. One circumferential end portion (hereinafter referred to as “front end 512a”) of each of the foils 512 and another circumferential end portion (hereinafter referred to as “rear end 512b”) of each of the foils 512 are inclined axially inward from both axial ends toward one circumferential side (downstream side in the rotation direction of the shaft) to form a so-called herringbone pattern. As illustrated in FIG. 54, the front end 512a of each of the foils 512 is formed as a free end so that a region including the front end 512a of each of the foils 512 functions as a top foil portion A′ having a bearing surface. A region including the rear end 512b of each of the foils 512 functions as a back foil portion B′ configured to support the top foil portion A′ of the adjacent one of the foils 512 from behind. Although a plurality of the foils 512 are coupled by a coupling portion in Patent Literature 5, the coupling portion is omitted in FIG. 53. In FIG. 53, only the three foils 512, which are arranged so that the foils 512 are slightly shifted in the axial direction (vertical direction in the figures), are illustrated for easy understanding.